1. Field of the Invention
This invention relates generally to a transfer case for an all-wheel drive (AWD)/four-wheel drive (4WD) vehicle and, more particularly, to a transfer case for an AWD/4WD vehicle that employs a planetary gear assembly, direct clutch assembly, and drum and band assembly that combine to provide overdrive or underdrive and direct drive so as to extend the driving gear ratios of the application.
2. Discussion of the Related Art
Various sport utility vehicles (SUV), off-road vehicles, four-wheel drive vehicles, etc. are equipped with drive modes and gear ranges that allow the vehicle to be driven in either high range or low range as well as one or more of two-wheel drive, four-wheel drive and AWD. Typically, these types of vehicles employ transfer cases having an input shaft that receives drive power from an output shaft of the vehicle's transmission. The input shaft distributes the drive power to a front output shaft that is coupled to a front drive shaft that drives the vehicle's front wheels and a rear output shaft that is coupled to a rear drive shaft that drives the vehicle's rear wheels. Known transfer cases have employed various types of couplings, such as viscous couplings, electromagnetic clutches, positionable spur gears, etc., that allow the drive power from the transmission to be distributed to the front and rear drive shafts to provide the various drive modes.
First generation transfer cases employed shift mechanisms that could only shift between the various drive modes when the vehicle was stopped. Typically, a four bar linkage was used to manually shift between drive modes using a mechanical shift actuator. Modern transfer cases sometimes employ synchronizers to synchronize the speed of the two output shafts of the transfer case prior to shifting between drive modes to allow drive mode shifts while the vehicle is moving (shift-on-the-fly). Some modem transfer cases employ synchronizers to synchronize the speed of the output shaft to either the input shaft or the output of the reduction gearset allowing range shifts to occur with the vehicle in motion as long as the transmission is shifted to neutral (shift-on-the-move).
In modern transfer cases, mechanical shift actuators have been replaced with electronically controlled shift actuators that are typically operated by an electric motor. In one known transfer case of this type, a reversible DC electric motor is employed to rotate a cammed shift actuator to selectively move two shift forks to establish the range and mode within the transfer case. The desired drive range and desired drive mode are selected by operating the motor under the control of a microprocessor-based control circuit. The microprocessor commands a motor drive circuit to energize the motor to run in either the clockwise or counter-clockwise direction to achieve the desired drive mode and range by stopping in the appropriate angular position.
An automatic vehicle transmission has a set gear range or overall ratio (OAR) that can be calculated by well known mathematical formulae for planetary or parallel shaft gearing that is designed for the desired performance of the vehicle. For example, vehicles that are designed to pull heavy loads may have a lower gear ratio range where deeper ratio gears are provided for more torque. Other vehicles may have a higher gear ratio range where lower numerical ratio gears are desired to achieve faster speeds and greater fuel economy.
Because the gear range of a transmission is set to achieve particular driving characteristics, there is no way to achieve greater torque or greater performance beyond what is available from the transmission. Therefore, it may be desirable in some designs to provide an additional underdrive or lower gear ratio and/or overdrive or higher gear ratio in combination with the set gear range of the transmission. For example, if the ratio of first gear in the transmission is 3:1 and the gear ratio of the added system is 2:1, then the overall gear ratio between the engine and the driveshafts connected to the front and rear axles would be 6:1.